The invention relates to a method for detecting blockages and interruptions in a pipe system of an aspirated smoke detector. The technical term for such detectors is ASD (Aspirated Smoke Detector).
The invention also relates to an aspirated smoke detector.
From the European published application EP 1 638 062 A1, a method is known for detecting blockages and interruptions in the pipe system of an aspirating fire detector, with which the fire detector simultaneously monitors air for characteristics of burning via the pipe system from one or more areas to be monitored or items of electrical equipment. The aspirating fire detector monitors a mass and/or volume flow determined by an air flow sensor and/or on the basis of current ventilator data by comparison with predetermined limit values. A correction value is determined, which represents changes in the properties of the system including the aspiration pipe and ventilator, based on changes in the density of the air and/or in at least one environmental parameter induced by a change in air density. This correction value is used to correct the mass and/or volume flow measured value and/or to adjust the limit values.
From the European published application EP 0 696 787 A1, a method is known for the detection of fires and gases in areas or in electrical or electronic equipment, with which a representative volume proportion of the spatial air or the cool air flow of the equipment unit is drawn off and conducted to a measurement chamber with at least one detector for the detection of characteristics of burning. The flow of the conducted air is monitored for changes. In addition, pressure fluctuations, in particular of the atmospheric air pressure, which take effect on the drawn off and conducted air are compensated for. To this effect the output signal from an air flow sensor is corrected by the output signal of a pressure sensor.
For the monitoring of the air flow, the air volume flow {dot over (V)} of the drawn off representative air portion quantity is significant. This should therefore remain as constant as possible. The air volume flow can be measured, for example, by a fan wheel anemometer or by a thermal air flow sensor, the output signal of which is a measure of an air mass flow {dot over (m)} which is approximately proportional to the air volume flow.
The mass flow {dot over (m)} and the volume flow {dot over (V)} are in this situation, as is known, linked to one another according to the physical relationship{dot over (m)}=ρ·{dot over (V)}  (I)by the density ρ. The latter is linked according to the physical relationship
                    ρ        =                  p                                    R              f                        ·            T                                              (        II        )            to the air pressure p, to the gas constant Rf for the air, and to the air temperature T. The density ρ is therefore proportional to the air pressure p and inversely proportional to the air temperature T. The gas constant Rf is dependent in a determinant manner on the air density, according to the following physical relationship:
                              R          f                =                              R            l                                1            -                          φ              ·                                                p                  d                                /                p                            ·                              (                                  1                  -                                                            R                      l                                        /                                          R                      d                                                                      )                                                                        (        III        )            where φ is the relative air humidity, Ri is the gas constant for dry air, Rd the gas constant of water vapor, p the ambient pressure, and pd the saturation vapor pressure of water in air.
The method described in EP 1 638 062 A1 in the introduction is however complex with regard to the determination of the correction value, wherein this correction value is intended to represent changes in the properties of the system including the aspiration pipe and ventilator, based on changes in the density of the air and/or at least one environmental parameter incurred by a change in the air density.
A further disadvantage lies in the inertia of the correction intervention, i.e. in the correction of the air volume flow measured value or air mass flow value respectively, issued by the air flow sensor, and in the correction of the upper and lower limits for the air volume flow and for the air mass flow respectively in the event of an interruption or a blockage.